The Sorcerer, Ghost Valley

The Sorcerer, Ghost Valley

Tuesday, 30 November 2010

Sling strengths



Some recent studies from working think tank of experienced climbers from around the world, have all agreed that ‘slings are great, their simple, lightweight & strong but try to avoid knotting the sling as this dramatically weakens it!
That’s all well and good, but 9 times out of 10 when we pull out a sling to use it, we tie a knot in it, eg to build a belay.
We’ve all been aware that knots weaken slings, but asked exactly by how much? and most people will only be able to give you a guesstimate.
Studies into the breaking strains of both Dyneema & Nylon slings with and without knot’s has been looked at before, but this group has done their own tests with the results below.

8mm Dyneema (av = average of 2 tests, -/+% = degradation or increased strength)
Normal tested sling strength          15cm           av 21.17kN      n/a
Normal tested sling strength          30cm           av 22.33kN      n/a
An overhand knot added                30cm           av 12.79kN      -42.73%
A Fig 8 knot added                         30cm           av 13.42kN      -39.89%
Larksfoot 2 slings together            2x15cm       av 14.50kN      -31.51%
Larksfoot on a 10mm pin               30cm           av 19.70kN      -11.76%
A Doubled Sling                              30cm           av 45.66kN      +104.21%



11mm Dyneema (av = average of 2 tests, -/+% = degradation or increased strength)
Normal tested sling strength          15cm             av 24.31kN      n/a
Normal tested sling strength          30cm             av 26.16kN      n/a
An overhand knot added                30cm             av 11.92kN      -54.43%
A Fig 8 knot added                         30cm             av 12.63kN      -51.73%
Larksfoot 2 slings                            2x15cm         av 12.66kN      -47.92%
Larksfoot on a 10mm pin               30cm             av 19.24kN      -26.44%
Doubled Sling                                  30cm             av 47.94kN      +83.29%



16mm Nylon (av = average of 2 tests, -/+% = degradation or increased strength)
Normal sling strength                        30cm             av 29.76kN      n/a
An overhand knot added                30cm             av 19.18kN        -35.57%
A Fig 8 knot added                         30cm             av 17.45kN        -41.36%
Larksfoot 2 slings                            2x15cm         av 23.64kN        -20.58%
Larksfoot on a 10mm pin               30cm             av 19.89kN        -33.17%

While the Dyneema slings have a greater strength to weight ratio with a static load than Nylon, it’s elasticity is far less
The results show all the slings weaken alarmingly once a knot is tied
It brings into question the sling & how best to use it?
You could argue that a sling knotted and used to join anchors together when equalising a belay severely weakens it. Add to this other forces such as angles & any shock loading to the system.... then another possibly more robust system could be used?
The best system then must be using the climbing rope to tie yourself in with. The rope will have an amount of stretch in it, making for a more dynamic belay, being softer on the gear you’ve placed and the knots you’ve tied.



This system is also very quick and super flexible when it comes to lengths and the ability to equalised different lengths easily that a fixed length sling no way can.
This works well when you’re alternating leads, but you need a second robust system if you are leading most of the pitches and want a simple changeover at the belay, an obvious clear point for the second to clip into and an easy of you quickly unclipped and heading off again?
For this system, most people will tend to use a single 8ft sling to join the anchors, equalising them, then tying a knot which then makes the anchors independent and also creating an obvious point for the second to clip into, which in most cases is a loop. If the anchors were further apart then two slings could be used and knotted in a similar way.
But with the recent results in mind, the best system using slings with regards strength has to be using two independent slings that join the two anchors together but without a knot. We don’t use this system very often because the anchors are always in varying distances apart and it not easy to equalise them exactly with fixed sling lengths and it’s also hard to get the direction of pull spot on.
So if you want to make the belay more dynamic then tie yourself into it and then belay from the rope loop on our harness (the loop created when you initially tied on). This allows for more elasticity in the system, which is softer on the slings and anchors.


But if you’re belaying direct or belaying two friends/climbers in parallel, at the same time, then using a rope sling is something worth thinking about? It’s stronger and being more elastic is softer on the gear you’ve placed to create an anchor eg ice screws.
I do know that one manufacturer is in the process of creating rope slings without the bulky knots, which is the downside of a rope sling and I guess one of the reasons slings have become so popular. They are stitching the rope ends together (one rope end laid over the top of the other) to try and get away with the bulky knot.
But until that point carrying a few rope slings could give you more options when building a safe belay.
You could also carry a length of cord on your harness say 10/12ft which you could use to join any number of anchors together of varying distances to help create a belay.




Metolius also manufacture a belay sling which is a number of small stitched loops linked together to create a chain. You can then link the anchors and equalise them using lengths of small stitched slings without the use of knots. The sling is heavier and more bulky but is very robust and strong when used to join anchors together in this way.
You could also use one of these rope slings to create a ‘cow’s tail’ when abseiling, which would also be a lot stronger and essentially more dynamic in any fall factor or slip, than a Dyneema or Nylon sling? You wouldn’t want to take a fall directly onto an 8ft sling with knot in it!


Slings have their place and always will do, over half of my quickdraws, whether I’m in the mountains or trad climbing are 4ft narrow tape Dyneema slings triple threaded to make my quickdraws but can be extended to lengthen the quickdraw to create less rope drag & so softer on the gear you place so not to lift them out or to avoid sideways pulls on the gear in the event of a fall.
Also slings are great to thread being less bulky and also light on the bigger mountain routes.

But if you do use a sling to build a belay then make sure there is no slack in the system & try to also build the rope into it giving the system more shock absorbency.
The recent results and discussions just give food for thought to how we use slings backed up by some test results in a certain situation. Slings are something we take for granted and it doesn’t hurt to keep revising our kit and how we use it.
Cheers
Ade

1 comment:

  1. Sorry to dredge up an old blog post, but this is a high result on Google and it's got some dangerous advice. Namely this:

    "But with the recent results in mind, the best system using slings with regards strength has to be using two independent slings that join the two anchors together but without a knot. We don’t use this system very often because the anchors are always in varying distances apart and it not easy to equalise them exactly with fixed sling lengths and it’s also hard to get the direction of pull spot on."

    That's not the only reason master points are formed with a knot. The other (and more important) reason is that clipping multiple slings coming in at different angles to a single carabiner will tri-load the carabiner. Carabiners are only full-strength when the direction of pull travels directly down the spine—when they're pulled three ways (left anchor, right anchor, and rope) they lose a dramatic amount of strength—more than knotting a sling.

    There's a reason we don't hear about knotted slings breaking in the real world. A 10 kN fall won't just break knotted slings, it will break your spine. Soft gear is rated to 22 kN because manufacturers (and CE and UIAA) expect them to be able to lose about 50% of their strength due to knotting and still hold any real-world climbing fall thrown at it. If we actually *needed* 22 kN of strength from a sling, they would be rated to 44 kN.

    ReplyDelete